Here's a hint which has worked well for me over the years. When you're done flying for the day, start the engine, advance the throttle to full, point the nose of the airplane straight down so that the clunk is out of the remaining fuel in the tank and wait until the engine quits. It'll take only a few seconds. This burns all the raw fuel that's in the engine, so all that's left is the oil residue. Then, there really is no need to use an afterrun oil, although it doesn't hurt to do so. It's the raw fuel left in an engine that causes the rusting and corrosion problems.

Frank T.